This invention relates to a proton exchange membrane (PEM) fuel cell, a direct methanol fuel cell (DMFC) and, more particularly, to the performance improvement of H2/air PEM fuel cells and direct methanol fuel cells.
Proton exchange membrane (PEM) fuel cells and direct methanol fuel cells are devices that convert the chemical energy of fuels, such as hydrogen and methanol, directly into electrical energy. The major components of a fuel cell include an anode, an electrolyte, and a cathode. The electrodes are usually made from noble metals such as platinum and platinum/ruthenium alloys and the electrolyte usually comprise a proton exchange membrane, such as Nafion(copyright) ionic-conducting, perfluorinated ionomer.
In H2/air fuel cells, H2 is oxidized at the anodes, O2 is reduced at the cathodes, and electricity is generated when electrons produced in the electrode reactions flow in the external circuit. The electrode reactions are:
Fuel Cell Anode: H2xe2x86x922H++2exe2x88x92
Fuel Cell Cathode: xc2xdO2+2H++2exe2x88x92xe2x86x92H2O
Overall Reaction: H2+xc2xdO2xe2x86x92H2O
In direct methanol fuel cells, the anode fuel is methanol and the corresponding electrode reactions are:
Fuel Cell Anode: CH3OH+H2Oxe2x86x92CO2+6H++6exe2x88x92
Fuel Cell Cathode: 3/2O2+6H++6exe2x88x92xe2x86x923H2O
Overall Reaction: CH3OH+3/2O2xe2x86x922H2O+CO2
The performance of a fuel cell is directly dependent upon the activities of the two electrodes, which can be improved by increasing catalyst surface area and loadings. However, increasing the catalyst surface area is not without its limitations. U.S. Pat. No. 5,480,851, issued to Tsurumi, et al on Jan. 2, 1996 for PROCESS OF PREPARING CATALYST SUPPORTING HIGHLY DISPERSED METAL PARTICLES, discloses a method for making high surface area supported platinum catalysts. The highest surface area achieved with this method was approximately 170 m2/g-platinum.
Improving electrode activities through increasing catalyst loadings also has its limitations. At low catalyst loadings, the activities of the electrodes increase with the increase of the catalyst loadings. The performance of a fuel cell levels off after the catalyst loadings reach a certain point. Further attempts to increase the catalyst loadings can have a negative effect upon the performance, causing it to decline by reason of increasing ohmic and mass transfer resistance.
In addition, electrode activities are affected by catalyst utilization. It is known that only a portion of catalyst sites is utilized because other sites are not accessible to the reactants. The inaccessibility to some catalyst sites can be due to dead pores or pores filled with water, whereby gas reactants such as O2 are prevented from reaching these catalyst sites. Improving catalyst utilization requires state of art skills. U.S. Pat. No. 5,501,915, issued to Hards et al on Mar. 26, 1996 for POROUS ELECTRODE FOR ELECTRODE ASSEMBLIES IN A FUEL CELL, discloses an electrode preparation method which can greatly improve catalyst utilization. The disclosed method uses two components (catalyst component and gas supply component), instead of the traditional one component, in catalyst preparation. The addition of the gas supply component decreases the loss associated with mass transfer and enhances the platinum utilization by more than 20%. However, this two-component method is very complicated and makes mass production difficult.
It has been discovered by the applicants that H2 evolution can significantly improve the performance of air cathodes and direct methanol fuel cell anodes. The improvement of air cathodes applies to both H2/air PEM fuel cells and direct methanol fuel cells. This electrochemical method is very easy to carry out. Furthermore, it does not require the complicated electrode preparation procedure as disclosed in U.S. Pat. No. 5,501,915.
The present invention pertains to a method and article operating according to the method that improves the performance of cathode electrodes used in H2/air PEM fuel cells and direct methanol fuel cells and anode electrodes used in direct methanol fuel cells. Fuel cell cathodes and anodes are often catalyzed with noble metals such as platinum and platinum/ruthenium alloys. It has been discovered that H2 evolution can significantly improve the performance of air cathodes in H2/air PEM fuel cells and direct methanol fuel cells and anodes in direct methanol fuel cells.
It is an object of this invention to provide a method that significantly improves the performance of H2/air PEM fuel cells and direct methanol fuel cells. This method is H2 evolution.